1. Field of the Disclosure
The present disclosure relates to a vertical external cavity surface emitting laser (VECSEL), and more particularly, to a VECSEL of a linear structure, certain embodiments of which being capable of achieving an excellent efficiency of a second harmonic generation (SHG) crystal and being manufactured to have a compact size.
2. Description of the Related Art
A VECSEL is a laser device providing a high output typically exceeding several to several tens of watts by replacing an upper mirror of a vertical cavity surface emitting laser (VCSEL) with an external mirror in order to increase a gain region.
FIG. 1 is a schematic sectional view of a prior art VECSEL having a linear structure. Referring to FIG. 1, the VECSEL 10 includes a laser chip 12 for laser oscillation attached on a heat spreader 11, an external mirror 15 spaced a predetermined distance from the laser chip 12, and a pump laser 16 obliquely disposed with respect to the laser chip 12 to provide a light for pumping to the laser chip 12. The laser chip 12 has a structure in which a distributed Bragg reflector (DBR) layer 12b and an active layer 12c are sequentially stacked on a substrate 12a. The active layer 12c has a multiple quantum well structure and is excited by a light for pumping to emit a light having a predetermined wavelength. The pump laser 16 allows a light of a wavelength shorter than that of a light that will be emitted from the laser chip 12, to be incident to the laser chip 12 through a lens 17 to excite the active layer 12c within the laser chip 12.
Also, a birefringent filter 13 passing only a light of a predetermined wavelength, and an SHG crystal 14 doubling the frequency of a light may be further located between the laser chip 12 and the external mirror 15. When the SHG crystal 14 is used, it is possible to convert an infrared light emitted from the laser chip 12 into a laser beam having a wavelength in a visible region.
With this structure, when a light beam emitted from the pump laser 16 is incident to the laser chip 12 through the lens 17, the active layer 12c within the laser chip 12 is excited to emit a light of a predetermined wavelength. The light emitted from the laser clip 12 resonates between the DBR layer 12b within the laser chip 12 and the external mirror 15. Then, the light whose frequency has been doubled by the SHG crystal 14 is outputted to the outside through the external mirror 15. To reduce light-loss, a coating layer may be formed on the surface 14a of the SHG crystal 14 that faces the laser chip 12 to have high reflectance with respect to a visible light and have high transmittance with respect to an infrared light.
The wavelength conversion efficiency of the SHG crystal 14 is generally proportional to the energy density of an incident light. Therefore, the beam diameter of a light incident to the SHG crystal 14 may be small, as small as possible. However, as illustrated in FIG. 1, since the prior VECSEL includes the SHG crystal 14 located far away from the laser chip 12, the beam diameter of an incident light is relatively large, which reduces the wavelength conversion efficiency of the SHG crystal 14.
To solve this problem, another prior VECSEL 20 illustrated in FIG. 2 includes an SHG crystal 24 located between a laser chip 22 and a birefringent filter 23. In FIG. 2, other elements, i.e., a heat spreader 21, the laser chip 22, an external mirror 25, a pump laser 26, and a lens 27 are the same as those described with reference to FIG. 1. With this structure, a light generated from the laser chip 22 resonates between a DBR layer 22b and the external mirror 25, and a light whose frequency has been doubled by the SHG crystal 24 is reflected and outputted to the outside by a birefringent filter 23. For that purpose, a coating layer is formed on the surface of the birefringent filter 23 to have high reflectance with respect to the light whose frequency has been doubled. According to the prior VECSEL in FIG. 2, since the SHG crystal 24 is located closer to the laser chip 22, the wavelength conversion efficiency of the SHG crystal 24 is enhanced even more.
However, even in this case, since the beam diameter of a light incident to the laser chip 22 should be equal to the size of the oscillation region of the laser chip 22, it is difficult to minimize the beam diameter of a light incident to the SHG crystal 24. Also, there is a problem that the light whose frequency has been doubled is outputted to the lateral side of the VECSEL 20 obliquely with respect to the VECSEL 20. Furthermore, parts of light having predetermined polarization directions and whose frequencies have not been doubled may be reflected and outputted to the outside by the birefringent filter 23.
FIG. 3 is a schematic sectional view of a VECSEL 30 of a folding structure to more enhance the efficiency of an SHG crystal. As illustrated in FIG. 3, according to the VECSEL 30 of the folding structure, a light generated from a laser chip 32 passes through a birefringent filter 33, and is obliquely reflected by a concave folding mirror 35 and propagates toward a flat external mirror 38. Therefore, the light resonates between a DBR layer 32b of the laser chip 32 and the external mirror 38, and a cavity is folded by the folding mirror 35. The SHG crystal 34 is located between the folding mirror 35 and the external mirror 38, and a light whose wavelength has been doubled by the SHG crystal 34 is reflected by the external mirror 38 and then outputted to the outside through the folding mirror 35. For that purpose, the surface of the external mirror 38 is coated to have high reflectance with respect to both a light whose frequency has been doubled and a light whose frequency has not been doubled. Also, the folding mirror 35 is coated to have high reflectance with respect to a light whose frequency has not been doubled and have high transmittance with respect to a light whose frequency has been doubled. In FIG. 3, a heat spreader 31, a pump laser 36, and a lens 37 are the same as those described with reference to FIGS. 1 and 2.
In this case, since the cavity is divided into two parts by the folding mirror 35, it is possible to control the beam diameter of a light incident to the laser chip 32 and the beam diameter of a light incident to the SHG crystal 34, respectively. Particularly, it is possible to optimize the wavelength conversion efficiency of the SHG crystal 34 by allowing a light to converge at the position of the SHG crystal 34.
However, in case of the VECSEL illustrated in FIG. 3, not only is one mirror further required but also the mirror should be disposed obliquely with respect to the laser chip, which makes it difficult to align parts. Therefore, the size of an entire laser system also increases. Also, in case of the VECSELs illustrated in FIGS. 1 through 3, the pump laser allowing the laser chip to oscillate is also disposed obliquely with respect to the laser chip, which makes it difficult to align parts and the size of the laser system increases.